Elastic metal filament yarn

ABSTRACT

An elastic metal filament yarn, compatible with any natural and/or synthetic filamentary yarn or composite material, consisting of a plurality of outer peripheral metal filaments and at least one central metal filament located along the axis of the filament yarn, where said outer filaments are helically twisted around the yarn axis defining a void therein, wherein the outer metal filaments will collapse toward the central filament when axial tension is applied to the yarn.

Unite States Patent Roberts et al.

[54] ELASTIC METAL FILAMENT YARN [72] Inventors: John A. Roberts, North Chelmsford; Peter R. Roberts, Groton; James E.

Kelly, Burlington, all of Mass.

' [73] Assignee: Brunswick Corporation 221 Filed: Feb. 24, 1971 21] Appl.No.: 118,320

[52] US. Cl. ..57/ 144, 57/ 160, 28/76 T [5 l] Int. Cl. ..D02g 3/12 [58] Field of Search ..57/34 R, 34 AT, 35, 140 BY, 57/144, 145, 146, 147,149,157 AS, 160,

[56] References Cited UNITED STATES PATENTS Clay ..28/76T Oct. 24, 1972 3,146,576 9/1964 Wezel ..57/160 3,378,999 4/1968 Roberts et a] ..57/145 X 3,522,699 8/ 1970 Popradi ..57/145 Primary Examiner-Wemer H. Schroeder Attorney-Donald S. Olexa, Jerome M. Teplitz, John G. Heimovics, William G. Lawler and Hofgren, Wegner, Allen, Stellman & McCord ABSTRACT An elastic metal filament yam, compatible with any natural and/or synthetic filamentary yarn or composite material, consisting of a plurality of outer peripheral metal filaments and at least one central metal filament located along the axis of the filament yarn, where said outer filaments are helically twisted around the yarn axis defming a void therein, wherein the outer metal filaments will collapse toward the central filament when axial tension is applied to the yarn.

12 Claims, 6 Drawing Figures ELASTIC METAL FILAMENT YARN FIELD OF THE INVENTION The present invention relates to a new and novel elastic metal multi-filament yarn for blending with and reinforcing natural and/or synthetic composites, fabric, and textile materials. For example, it could be used with any composition of organic material used for fabric in clothes, for use in fuel hose lines, as well as being directly used as tire cord. The metal yarn incorporates a plurality of outer filaments helically twisted around a central filament, defining a void therebetween, such that upon breaking the central filament the metal filament yarn is highly elastic under low tension, by translating toward the center until reaching the central filament, after which further tension will induce high stress and low elongation.

In particular the idea of the present invention provides for the ability to predeterminatively control the characteristics of elongation in the metal filament yarn by preselectively arranging the metal filaments in the outer region of a filament yarn matrix surrounding a sacrificial inner matrix material subsequently removed therefrom with the sacrificial or expendable inner matrix material having a centrally located non-expendable filament therein, such that any desired percentage of geometric elongation with the optimum mechanical properties of the filament yarn may be obtained. In this way any specified amount of geometric elongation can be designed into the metal yarn to make it compatible with the elongation characteristics of any particular composition of natural and/or synthetic yarn or fabric. By geometric elongation and elasticity is meant reversible changes in length of the filament yarn due to a change in geometry of the configuration in which the filament yarn is set.

DESCRIPTION OF THE PRIOR ART One of the major problems in using single strand metal filament with natural or synthetic yarn is the lack of stress and elongation compatibility between the metal and organic filaments. It is well known that organic filaments and fabric undergo a great amount of elongation at low tension levels. Alternatively, a single metal filament is known to have little elongation at similar tension levels. It would be desirable therefore to attempt to incorporate the physical properties of metal filaments as a reinforcing element in organic fabric for improved strength and wear qualities, crush characteristics, and better pile structure, taking advantage of both the strength and wear properties of the metal filaments and the high elongation and soil-hiding properties of organic material. However, attempts at combining single and multiple metal filaments with organic yarn have met with little success, not only because of elongation incompatibility, but also because the fabrication process manipulating the metal filament results in a material with a loose metal filament weave that gains, little advantage from the metal filament physical properties.

A few patents describing different embodiments of coreless yarn having a certain amount of elasticity are U.S. Pat. Nos. 3,399,521; 3,090,189; 3,189,499; 3,344,596; and 2,587,1 17. Some of these describe using elastic and inelastic threads for the yarn core, others talk of texturizing or processing the yarn to obtain elasticity by a volume increase, and still others teach using-prestressed elastic threads for the yarn core surrounded by a layer of inelastic outer strands. However, nowhere has the prior art shown attempts or ability to predeterminatively design and control the amount of elongation, density, and/or weight of the resulting elastic yarn. It was desired in the manufacture of elastic yarn to be able to design into the cored yarn those elongation characteristics necessary to suit any particular composition of natural and/ or synthetic fibers.

Although desirable, no prior art yarn or processes for making them, have the ability to predeterminatively design into the yarn those optimum physical properties and characteristics making it compatible in geometric elongation, density, and weight (and any variation thereof), with natural and/or synthetic fabrics.

SUMMARY OF THE INVENTION The present invention then is generally intended to overcome the deficiencies of the prior art and to provide for a high strength metal filament yarn geometrically elastic at tension levels normally encountered in natural or synthetic filaments or fabric.

It is accordingly a general object of the present invention to provide for the ability to predeterminatively control the geometric elongation characteristics in elastic metal yarn by a preselective arrangement of metal filaments in the outer region of a filament yarn matrix.

Another object of the present invention resides in the provision of forming an elastic metal yarn, having its outer filaments equally stressed upon application of axial tension, by preselective arrangement of the outer filaments in a filament yarn matrix, thereby optimizing the maximum strength of elongation of the filament yarn.

A further object of the present invention resides in the provision of an elastic metal yarn having elongation characteristics compatible with the elongation characteristics of natural and/or synthetic yarn or fabric.

A feature of the resent invention provides for a metal filament yarn matrix that can have a preselected filament yarn to sacrificial matrix material ratio and filament arrangement thereby permitting design of the exact filament density and weight desired in the outer region of the resulting elastic metal yarn.

Another feature of the present invention resides in the provision of utilizing a central unstressed metal filament in the yarn that is manipulated during the sewing process thereby eliminating the problem of stretching of the elastic filament yarn during the manufacturing process, the central filament being broken after material fabrication thereby permitting the metal yarn to compatibly stretch with the natural and/or synthetic fabric.

A further feature of the present invention provides for an elastic metal filament yarn that can be compatibly piled with natural and/or synthetic fiber or material as a reinforcing element to improve the strength and wear qualities or the natural and/or synthetic material.

Another feature of plying the elastic metal filament yarn of the present invention with natural and/or synthetic material resides in the ability to control and eliminate any static electricity developed in the natural and/or synthetic material.

The novel features which are believed to be characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, together with further objects and advantages thereof will best be understood by reference to the following description taken in connection with the accompanying drawings, in which:

FIG. 1 is a perspective view of the filament yarn embodying the invention;

FIG. 2 is a cross-section of the filament yarn under no axial tension, showing the general arrangement of the outer filaments preselectively arranged around a sacrificial inner matrix and centrally located filament;

FIG. 3 is a cross-section of the filament yarn under axial tension showing the general arrangement of the filaments after they have collapsed through a central void left after removing the sacrificial inner matrix;

FIG. 4 is a photomicrograph at 50X magnification showing a final configuration of the filament yarn matrix, as described in FIG. 3, after hot forming and cold drawing but prior to leaching out of the sacrificial inner matrix material;

FIG. 5 is a photomicrograph at 200 magnification showing a final configuration of another arrangement of a filament yarn matrix, prior to leaching out the sacrificial inner matrix material; and

FIG. 6 is a filament identification code.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the preferred embodiment of the present invention, a metal filament yarn generally designated 10 of filaments 12, as shown in FIG. 1, is formed by a process described in detail in U.S. Pat. No. 3,394,213, and U.S. Pat. No. 3,505,039. Broadly, these inventions comprehend the constriction of the bundled wires, by first forming the bundled wires or elements forming constrictions and subsequent cold drawing constrictions. The hot forming constrictions may be alternatively hot extrusion or hot rolling of the billet. The drawing operation may comprise a plurality of cold drawing steps with intermediate annealing steps.

The metal filament yarn of the present invention then is the product of appropriate bundle-drawing,

twisting and leaching process of a metallic yarn matrix with a hollow core being developed and containing at least one central metal filament, the overall configuration of this yarn matrix being such, that the yarn structure is capable of a predetermined amount of geometric elongation before the individual filaments become highly stressed.

Referring to FIG. 2, the filament yarn 10 is initially formed in a sheath 22 which generally consists of a leachable filament matrix material 16 and non-leachable outer filaments 12. It will be noticed that there is at least one central non-leachable filament 14 which is necessary to the functioning of the present invention. The filaments 12 of the metal filament yarn 10 are further helically twisted with respect to a central filament 14 located along the longitudinal axis of filament yarn 10. This helical twist being set into the outer filament 12 during one of the last drawing steps as described in the prior inventions referred to. This results in the outer filaments 12 being held in spaced concentric helices around the matrix material 16 until the matrix material is removed in the leaching step,

which can be chemical attack of the sheath 22 and matrix material 16. Obviously, other methods of removing the sheath and matrix material may be employed, such as those described in the inventions referred to.

Around central filament 14 is arranged in matrix form a plurality of leachable filaments 16, that will leave a core or void 13 as shown in FIG. 1, between the non-leachable filaments l2 and central filament 14 after the leaching process. It should further be noted, that the non-leachable filaments 12 are arranged in substantially the. outer region or periphery of the metal filament yarn matrix 10. Dispersed within this outer ring of non-leachable filaments 12 as shown in FIGS. 2, 3, 4, and 5, are leachable spacers 20 similar to filaments 16. The leachable filaments 16 are for the purpose of providing for uniform deformation of the filaments 12 and central filament 14 during the processing step and room for movement of filaments 12 after leaching, so that all filaments 12 will collapse and stretch approximately the same amount during axial tension of the filament yarn. By rearranging the leachable filaments 16 in the inner region of the metal filament yarn 10, the ultimate tensile strength, density and weight can be varied and predetermined in the final resulting filament yarn. The spacer filaments 20 fill up the space in sheath 22 not occupied by the filaments 12, 14, and 16 in the hexagonal array, to assure uniform deformation of the filaments l2, l4, and 16 during processing takes place as described in the patents referred to earlier.

It has been determined theoretically and proven by experiments, that allowing for changes in filament diameter after leaching, and changes in filament position and number of twists per inch after stretching, it can be shown that:

where:

R, the initial helix radius, in units of filament diameter, defining the initial filament diameter position when in an untensioned state; R the final helix radius, in units of filament diameter, defining the final filament diameter position when mi rtsnsim ,7

L1 L0 E= =geometrie elongation of the helix from L0 to L L original filament yarn length, L final filament yarn length, D initial filament diameter, D, filament diameter after leaching, n initial number of twists per inch, .4? 2. a

Referring to FIG. 6, the radius R of the helix, denoting the positions A, B, C, D, etc. in the hexagonal array in Table I, is taken as Ro when referring to the filaments 12 when in an untensioned state as shown in FIGS. 1 and 2, and the radius R is taken as R, when referring to the filaments 12 when under tension as shown in FIG. 3.

The values of R and R, therefore, give the locations of filaments in a filament yarn matrix, as shown in the filament identification code given in FIG. 6, before and after stretching, for a given size, elongation, and initial helical twist. Some values for R and R are shown in a portion of the hexagonal array Table l, as examples, for the variety of filament positions. The initial filament diameter, D includes the leachable cladding material sheath around the filament 12, (shown in FIG. 2 as an example) as described and shown in the patents referred to earlier.

TABLE 1 R and R, For Various Positions in Hexagonal Array Where R X( Initial Filament Dia.) Initial Radius of Helix Where R X( Final Filament Dia.) Final Radius of Helix Position Position Position Position R,,orR RqOl'R R orR, R orR, A =0.0000 F1=4.5826 H|=6.5574 g, ='S. 6 Q3 B= 1.0000 F==5.0000 H=7.0000 K ==8.7178 C, 1.7371 G =5.l962 I =7.0000 K;,=8.8882 C= 2.0000 G2=5.29l5 h=7.2lll J =9.0000 D, =2.6457 G1=5.5677 I1=7.5498 K==9.l652 D 3.0000 Hs=5.5677 J4=7.8l02 Kr=9.5394 E =3A6l0 G=6.0000 J3=7.9372 L5=9.5394 E, 3.6055 I4=6.0827 I=8.0000 I .,=9.6436 =4 00 H =6.2450 Jz=8.l.853 I =9.8488 F.=4.3589 aatinthawq s.=

Since the resulting outer filaments 12 will be of unequal distance from the central filament l4, axial tension of the filament yarn will induce unequal loading of the individual filaments 12. This becomes very serious when there is a high degree of twist, for example turns per inch or more, resulting in the outer filaments 12a being much longer than the outer filaments 12b which are closer to the center of the filament yarn. The outer filaments 12a will therefore become loaded only after the closer filaments 12b have been stressed beyond their yield point or have been broken. Ideally, all outer filaments 12 should be loaded equally and simultaneously. The unequal loading problem due to both the unequal distance of the filaments 12 from the filament axis and the high degree of twist given to the filament yarn 10, can be minimized according to the equation given, and thereby optimize combinations of strength and elongation in the finished filament yarn 10. This can be obtained by a detailed calculation of the starting billet configuration, by first selecting a desired elongation E, for given diameters D and D and calculating R, values for various R locations and degrees of twist. By matching these R, values with R values of the standard hexagonal array (see Table 1), an initial billet configuration can be determined. Because of the large number of filaments in any useful filament yarn design, it is not possible to conform precisely in all cases to the calculated R position for each filament. For this reason, adjustments in degree of twist may be necessary to achieve optimum strength and elongation properties in the final yarn.

Several different billet configurations were calculated and processed according to the equation given, directed to producing uniformity of loading in the outer filaments 12 of the finished filament yarn -10, and

mechanical tests of the resultant filament yarn showed elongation in the ranges predicted, with substantially equal load distribution among the outer filaments 12 during geometric elongation.

FIG. 2 shows the arrangement of the outer filaments 12 in the filament yam 10, in an unstressed state. Application of tension will cause the filaments 12 to move through the void 13 (see FIG. 1) defined after the leachable filaments 16 are removed. FIG. 3 shows the filament yarn 10 under axial tension and depicts the hexagonal arrangement of the filaments 12 in their most dense configuration, this configuration optimizing the strength of the filament yarn 10 by equalizing the tension on the individual filaments 12.

One example of the formation of filament yarn of the present invention is as follows. The filaments 12c comprised 304 stainless steel wire 0.080 inch in diameter. The sheath 150 was a Monel 400 tube having a 0.1 15 inch outside diameter, and a 0.100 inch inside diameter. The case 22c was formed of mild steel having a 1.221 inch outside diameter and 1.059 inch inside diameter. One hundred and fifty six (156) of the sheathed filaments 12c, 154 of the leachable filaments 16c, and one central filament were placed in the case 220 in the arrangement shown in FIGS. 2 and 5, with proper spacers 20c and the case 220 evacuated to less than 0.1 microns of mercury at 800 degrees F. and sealed off. The billet or filament yarn matrix 10c was then heat treated at 1,800 degrees F. for six hours in a graphite container. The resulting composite was reduced in diameter 17X by extrusion and subsequently cold drawn to a final diameter with intermediate anneals (2 sec/mil at 1,800 degrees F.) as required. After twisting in the final draw to 0.0225 inch diameter with 10 turns per inch, the Monel matrix was leached out using standard procedures, to form the filament yarn, having an average geometric elongation of 20 percent and an average ultimate tensile strength of approximately 21,000 p.s.i.

The case 22 and matrix material 16 was removed by means of nitric acid.

Another example of the forming of filament yarn of the present invention comprises the following. Here the filaments 12d were 304 stainless steel wires 0.080 inch in diameter. The sheath 15d was a Monel 400 tube having a 0.115 inch outside diameter and a 0.100 inch inside diameter. The case 22d was formed of mild steel having a 2.00 inch outside diameter and 1.838 inch inside diameter. Five hundred and seventy-one of the sheathed filaments 12d (shown in FIG. 4), 569 of the leachable filaments 16d, and one central filament 14d were placed in case 22d in the arrangement shown in FIG. 4, with proper spacers 20d and the case 22d evacuated to less than 0.1 microns of mercury at 800 degrees F. and sealed off. The resulting billet or filament yarn matrix 10d was then heat treated at 1,800 degrees F. for 6 hours in a graphite container. The resulting composite was reduced in diameter 16X by extrusion and subsequently cold drawn down to final diameter with intermediate anneals (2 sec/mil at 1,800 degrees F.) as required. After twisting in the final cold draw to 0.40 inch diameter with 9 turns per inch, the Monel matrix was leached out using standard procedures, to form the filament yarn, having an average elongation of 20 percent and an average ultimate tensile strength of approximately 45,815 p.s.i.

The two examples of the above described yam matrix configurations prior to leaching of the expendable matrix material, are shown in FIGS. 4 and 5. FIG. 4 is a photomicrograph at 50 magnification of a crosssection of a filament yarn matrix in a sheath 22d having 570 stainless steel outer filaments 12d and one central filament Md. The expendable matrix filaments 16d, as can be seen, have somewhat become uniform during the processing of the filament yarn matrix. This is also true of the expendable spacer filaments 20a. FIG. is a photomicrograph at 200X magnification of a cross-section of a filament yarn matrix in a sheath 220 having 156 outer filaments 120, one central filament 14c, and a uniform distribution of expendable matrix filaments 16c. The expendable spacer filaments 20c, have also fused together somewhat.

As indicated briefly above, the present invention comprehends the idea of arranging a filament matrix having at least one central metal filament surrounded by a plurality of helically twisted filaments in the periphery of the filament matrix with a void therebetween left upon leaching out the leachable inner matrix material.

Not only can the filament yarn of the present invention be predesigned for compatibility with any composition of natural and/or synthetic yarn or fabric, the filament yarn can also be directly used in tires as a reinforcing tire cord. Finally, being electrically conductive, the metal filament yarn permits the control of static electricity developed in the material in which it is used. One example of this is using it in fuel hose lines where static electricity is developed by the movement of fuel through the hose. The metal filament yarn in this case can conduct the electricity to ground eliminating the hazard of electrical arcing and possible fuel ignition.

According to the equation given, the experimental results showed that a higher twist-per-inch will give a greater degree of geometric elongation. For a given configuration of filaments 12 and for a given size of initial filament diameter, D of filaments 12, there is an optimum twist per inch that will give optimum properties to the filament yarn 10. For a given number of turns per inch, the larger the initial starting metal filament matrix, the greater amount of geometric elongation. Higher ultimate tensile strengths can be obtained through cold working the filament yarn matrix prior to twisting, but with a corresponding reduction in percent elongation.

While we have shown and described specific embodiments of the present invention, it will, of course, be understood that other modifications and alternative constructions may be used without departing from the true spirit and scope of this invention. We therefore intend by the appended claims to cover all such modifications and alternative constructions as fall within their true spirit and scope.

What we intend to claim and secure, by Letters Patent of the United States, is:

1. An elastic filament yarn comprising:

a plurality of outer filaments preselectively arranged in substantially the peripheral region of the filament yarn and further defining a void therein, said outer filaments helically twisted with respect to the central axis of said void; and

at least one central filament centrally located along the central axis of said void;

wherein the central filament is breakable upon application of sufficient axial tension and the outer filaments are reversibly moveable through the void toward the central filament thereby permitting the filament yarn to be reversibly geometrically elongatable.

2. An elastic filament yarn as recited in claim 1 wherein the outer filaments and central filament are made of metal.

3. An elastic filament yarn as recited in claim 2 wherein the central filament can be stressed to its ultimate tensile stress and broken prior to stressing the outer filaments to their tensile yield stress.

4. An elastic filament yarn as recited in claim 1 wherein the plurality of outer filaments are preselectively arranged in the filament yarn prior to being helically twisted around said central axis, such that upon application of axial tension along said filament yarn the outer filaments will geometrically elongate uniformly and be substantially equally stressed.

5. A yarn having a plurality of filaments comprising:

at least one central filament centrally located along the central axis of said yarn and breakable upon application of sufficient axial tension;

an expendable matrix material surrounding said central filament; and plurality of outer filaments surrounding said ex-- pendable matrix material in a preselected arrangement and helically twisted with respect to the central filament, said outer filaments substantially defining the peripheral region of said yarn, wherein the expendable matrix material is removeable from the yarn, resulting in a yarn having a void left between the outer filaments and central filament defining space through which the outer filaments can reversibly move toward the central axis yarn application of sufficient axial tension to break said central filament.

6. A filament yarn as recited in claim 5 wherein the expendable matrix material is made of a leachable metal material and the outer filaments and central filament are made of a non-leachable metal material.

7. A filament yarn as recited in claim 6 wherein said outer filaments are preselectively arranged in the peripheral region of said yarn matrix prior to being helically twisted around said central axis.

8. A textile material comprising:

an elastic metal filament yarn plied with organic yarn, said metal filament yarn comprising;

a plurality'of outer filaments preselectively helically twisted filament yarn and defining a void therein; and

at least one central filament centrally located along the central axis of said void;

wherein said elastic metal filament yarn has geometric elongation properties substantially equal to the elongation properties of the organic yarn, thereby making them compatible with each other.

9. A textile material as recited in claim 8 wherein said elastic metal filamentary yarn has geometric elongation properties less than the elongation properties of the organic filamentary yarn.

10. A textile material as recited in claim 8 wherein said elastic metal filamentary yarn has geometric elongation properties greater than the elongation properties of the organic filamentary yarn.

l 1. An elastic yarn comprising: 12. A plurality of helically twisted exterior filaments a plurality of helically twisted flexible resilient exteriwherein the internal surface thereof defines a central or filaments wherein the internal surface thereof passage parallel to the axis of the yarn; defines a central core passage parallel ith h i at least one central core filament in the passage in a of the yarn; spaced relationship with respect to the internal at least one central core filament in the passage in a Surface Ofthe helical filaments; d

a matrix material substantially surrounding the core filament and filling the passage, the matrix material capable of being sacrificially removed from the helical filaments and the core filament.

spaced relationship with respect to the internal surface of the filaments; and

means for preventing the helical filaments from becoming untwisted when an axial load is applied thereto. 

2. An elastic filament yarn as recited in claim 1 wherein the outer filaments and central filament are made of metal.
 3. An elastic filament yarn as recited in claim 2 wherein the central filament can be stressed to its ultimate tensile stress and broken prior to stressing the outer filaments to their tensile yield stress.
 4. An elastic filament yarn as recited in claim 1 wherein the plurality of outer filaments are preselectively arranged in the filament yarn prior to being helically twisted around said central axis, such that upon application of axial tension along said filament yarn the outer filaments will geometrically elongate uniformly and be substantially equally stressed.
 5. A yarn having a plurality of filaments comprising: at least one central filament centrally located along the central axis of said yarn and breakable upon application of sufficient axial tension; an expendable matrix material surrounding said central filament; and a plurality of outer filaments surrounding said expendable matrix material in a preselected arrangement and helically twisted with respect to the central filament, said outer filaments substantially defining the peripheral region of said yarn, wherein the expendable matrix material is removeable from the yarn, resulting in a yarn having a void left between the outer filaments and central filament defining space through which the outer filaments can reversibly move toward the central axis yarn application of sufficient axial tension to break said central filament.
 6. A filament yarn as recited in claim 5 wherein the expendable matrix material is made of a leachable metal material and the outer filaments and central filament are made of a non-leachable metal material.
 7. A filament yarn as recited in claim 6 wherein said outer filaments are preselectively arranged in the peripheral region of said yarn matrix prior to being helically twisted around said central axis.
 8. A textile material comprising: an elastic metal filament yarn plied with organic yarn, said metal filament yarn comprising; a plurality of outer filaments preselectively helically twisted filament yarn and defining a void therein; and at least one central filament centrally located along the central axis of said void; wherein said elastic metal filament yarn has geometric elongation properties substantially equal to the elongation properties of the organic yarn, thereby making them compatible with each other.
 9. A textile material as recited in claim 8 wherein said elastic metal filamentary yarn has geometric elongation properties less than the elongation properties of the organic filamentary yarn.
 10. A textile material as recited in claim 8 wherein said elastic metal filamentary yarn has geometric elongation properties greater than the elongation properties of the organic filamentary yarn.
 11. An elastic yarn comprising: a plurality of helically twisted flexible resilient exterior filaments wherein the internal surface thereof defines a central core passage parallel with the axis of the yarn; at least one central core filament in the passage in a spaced relationship with respect to the internal surface of the filaments; and means for preventing the helical filaments from becoming untwisted when an axial load is applied thereto.
 12. A plurality of helically twisted exterior filaments wherein the internal surface thereof defines a central passage parallel to the axis of the yarn; at least one central core filament in the passage in a spaced relationship with respect to the internal surface of the helical filaments; and a matrix material substantially surrounding the core filament and filling the passage, the matrix material capable of being sacrificially removed from the helical filaments and the core filament. 